Polymerization of novel methacrylated anthraquinone dyes

Article abstract of DOI:10.3762/bjoc.9.48

A new series of polymerizable methacrylated anthraquinone dyes has been synthesized by nucleophilic aromatic substitution reactions and subsequent methacrylation. Thereby, green 5,8-bis(4-(2-methacryloxyethyl)phenylamino)-1,4- dihydroxyanthraquinone (2), blue 1,4-bis(4-((2-methacryloxyethyl)oxy) phenylamino)anthraquinone (6) and red 1-((2-methacryloxy-1,1- dimethylethyl)amino)anthraquinone (12), as well as 1-((1,3-dimethacryloxy-2- methylpropan-2-yl)amino)anthraquinone (15) were obtained. By mixing of these brilliant dyes in different ratios and concentrations, a broad color spectrum can be generated. After methacrylation, the monomeric dyes can be covalently emplaced into several copolymers. Due to two polymerizable functionalities, they can act as cross-linking agents. Thus, diffusion out of the polymer can be avoided, which increases the physiological compatibility and makes the dyes promising compounds for medical applications, such as iris implants.

Full text of DOI:10.3762/bjoc.9.48

Polymerization of novel methacrylated
anthraquinone dyes
Christian Dollendorf, Susanne Katharina Kreth, Soo Whan Choi
and Helmut Ritter*§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 453–459.
Institute for Organic and Macromolecular Chemistry II,
Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, 40225
Düsseldorf, Germany
doi:10.3762/bjoc.9.48
Received: 04 October 2012
Accepted: 25 January 2013
Published: 28 February 2013
Email:
Helmut Ritter* - H.Ritter@uni-duesseldorf.de
Associate Editor: D. O'Hagan
* Corresponding author
§ Tel.: +49 (0)211-81 14760; fax: +49 (0)211-81 15840
© 2013 Dollendorf et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
anthraquinone; monomeric dyes; red; green; blue dyes; polymer
Abstract
A new series of polymerizable methacrylated anthraquinone dyes has been synthesized by nucleophilic aromatic substitution reac-
tions and subsequent methacrylation. Thereby, green 5,8-bis(4-(2-methacryloxyethyl)phenylamino)-1,4-dihydroxyanthraquinone
(2), blue 1,4-bis(4-((2-methacryloxyethyl)oxy)phenylamino)anthraquinone (6) and red 1-((2-methacryloxy-1,1-
dimethylethyl)amino)anthraquinone (12), as well as 1-((1,3-dimethacryloxy-2-methylpropan-2-yl)amino)anthraquinone (15) were
obtained. By mixing of these brilliant dyes in different ratios and concentrations, a broad color spectrum can be generated. After
methacrylation, the monomeric dyes can be covalently emplaced into several copolymers. Due to two polymerizable functionalities,
they can act as cross-linking agents. Thus, diffusion out of the polymer can be avoided, which increases the physiological compati-
bility and makes the dyes promising compounds for medical applications, such as iris implants.
Introduction
Anthraquinone and its derivatives bearing hydroxy and amino these dyes are more complicated, their production costs are
moieties are of remarkable importance in pharmacological, higher in comparison to azo dyes. Therefore, anthraquinone
biochemical and dye industries [1,2]. They have been described dyes are only used if the required properties and colors are
as important compounds for decades [3-6] and are used for the remarkable or when the desired colors cannot be obtained easily
coloration of cotton and cellulose fibers as well as for synthetic by the use of azo dyes, especially in the case of bright blue
materials such as polyamides [7]. Besides azo dyes, they repre- shades. Anthraquinone dyes can be obtained synthetically and
sent the second largest class of textile pigments [8], and unlike by isolation from fungi such as Dermocybe sanguinea or other
these dyes, anthraquinone derivatives are resistant against de- fungal species [5,10-13], to give them a higher compatibility
gradation due to their aromatic structure [9]. As the syntheses of with the environment and an improved availability [14-16].
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Beilstein J. Org. Chem. 2013, 9, 453–459.
Coloration of polymers is usually achieved by mixing dyes or behavior of 2 compared to 5,8-dichloro-1,4-dihydroxyan-
pigments with (other) polymeric ingredients. In many cases, the thraquinone (λmax = 500 nm) is a result of the enlargement
colors rapidly fade or change as well as lose their mechanical of the π-electron system as aromatic moieties are introduced.
properties when exposed to sunlight. More stable polymers can This leads to absorption at longer wavelengths (bathochromic
be prepared by the reaction of polymerizable dyes with appro- shift). Thus, the challenge of creating a green dye is to realize
priate comonomers. In the past, these materials have been selective absorption in two different areas of the visible spec-
widely used because of their photo- and thermostability, their trum.
strong fluorescence, or their resistance to water and other
Polymerizable blue dye 6 based on
solvents [7,17,18].
anthraquinone
The purpose of this work was to create dyes of brilliant color For the synthesis of the blue anthraquinone based monomeric
modified with polymerizable groups, to enable the preparation dye 6, the precursor 1,4-dichloroanthraquinone (3) was
of polymeric materials with covalently emplaced dyes. A broad prepared by the reaction of 1,4-dichlorobenzene with phthalic
range of the color spectrum can be produced by mixing three dichloride by Friedel–Crafts acylation followed by an acidic
colors such as blue, green and red, which were synthesized in ring closure (Scheme 1, step b). In further reaction steps, 1,4-
this work [19-24]. For example, colored polymers can be used dichloroanthraquinone (3) and hydroxyaniline were reacted to
for medical applications such as iris implants, where damaged obtain 1,4-bis(4-hydroxyphenylamino)anthraquinone (4).
irides are replenished with polymeric replications [25-27].
Subsequent reaction with 2-bromoethanol gave 1,4-bis(4-((2-
hydroxy-ethyl)oxy)phenylamino)anthraquinone (5). In order to
introduce polymerizable functionalities, 5 was treated with
Results and Discussion
Starting from commercially available anthraquinone-deriva- methacrylic chloride to give 1,4-bis(4-((2-methacryloxy-
tives, various polymerizable dyes were synthesized. Three ethyl)oxy)phenylamino)anthraquinone (6) (Scheme 1, step c).
colors (green, blue, red) were obtained (Scheme 1).
The obtained blue chromophore 6 shows absorption maxima at
Polymerizable green dye 2 based on
anthraquinone
408, 600 and 644 nm in the UV–vis spectrum (Figure 1b).
These maxima are located in the purple, orange and red color
The synthesis of the green anthraquinone based monomeric dye ranges, which results in combination to give the color blue.
2 is shown in Scheme 1, step a. 5,8-Dichloro-1,4-dihydroxyan- Analogous to the green dye, methacrylation of 5 does not influ-
thraquinone was first subjected to a nucleophilic substitution ence the absorption maxima.
reaction with 2-(4-aminophenyl)ethanol to give a green-colored
Polymerizable red dyes 9, 12 and 15 based
double-substituted intermediate 5,8-bis(4-(2-hydroxy-
ethyl)phenylamino)-1,4-dihydroxyanthraquinone (1) in 50% on anthraquinone
overall yield. According to the literature, reactions can be accel- A red monomeric dye 9 with non-aromatic amines as func-
erated with improved yields by microwave assistance [28,29]. tional moieties, which showed similar properties, was also
After microwave-assisted methacrylation with methacrylic synthesized starting from anthraquinone. 1,1-Dimethyl-2-
anhydride the polymerizable anthraquinone based monomeric hydroxyethylamine exhibits two methyl groups in the α-pos-
dye 2 was obtained. Due to steric reasons, methacrylic anhy- ition to the nitrogen atom, which shields the chromophoric
dride is less reactive than the corresponding chloride and there- amine. The absence of hydrogen atoms in the α-position also
fore selective for the methacrylation of the aliphatic hydroxy protects the amino group from further reactions. In the cases of
groups [25]. In this case, the use of methacryloyl chloride the green and blue monomeric dyes this effect was achieved by
always leads to the formation of several side-products. As introducing a benzene ring in the α-position [25].
cross-linking agent, 2 can be copolymerized with other
methacrylate- or acrylate-comonomers, to build up macromole- 1,4-Dichloroanthraquinone was treated with 1,1-dimethyl-2-
cules bearing covalently emplaced dye derivatives.
hydroxyethylamine to form two products, namely 1-chloro-4-
((2-hydroxy-1,1-dimethylethyl)amino)anthraquinone (7) and
The UV–vis spectra of 1 and 2 (Figure 1a) in each case show 1,4-bis((2-hydroxy-1,1-dimethylethyl)amino)anthraquinone (8).
three absorption maxima (416, 636 and 684 nm), which can be Both products can be separated by column chromatography and
found in the violet and red color ranges. A combination of these esterified with methacryloyl chloride to obtain polymerizable
two colors appears purple, the complementary color to green. monomeric dyes 9 and 10 (Scheme 1, step d). The monosubsti-
Methacrylation of 1 does not change the chromophore and tuted product 9 shows a characteristic red color and an absorp-
therefore the absorption maxima. The change in the absorption tion maximum at 506 nm, whereas the disubstituted product 10
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Beilstein J. Org. Chem. 2013, 9, 453–459.
Scheme 1: Synthesis of red (9, 12, 15), blue (6, 10) and green (2) polymerizable dyes.
is colored blue with absorption maxima at 574 and 618 nm 1-((1-hydroxy-2-methylpropane-2-yl)amino)anthraquinone (11)
(Figure 2).
in good yields (60%). Subsequent esterification with methacryl-
oyl chloride gives 12 [26,27]. This dye shows a characteristic
The synthesis of a red polymerizable dye 12 without any side- red color with an absorption maximum at 506 nm, which is
products can be accomplished by the use of 1-chloroan- located in the green color range, the complementary color to
thraquinone and 1,1-dimethyl-2-hydroxyethylamine giving red.
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Beilstein J. Org. Chem. 2013, 9, 453–459.
Figure 1: Visible spectra of the polymerizable dyes green 1/2 (a) and blue 6 (b).
Figure 2: Visible spectra of red 9 and blue 10.
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Beilstein J. Org. Chem. 2013, 9, 453–459.
By introduction of a second polymerizable functionality, diffu- (THFMA) and ethylene glycol dimethacrylate (EGDMA) as
sion of incompletely polymerized dyes out of the polymer cross-linking agents in order to obtain materials with duro-
network can be avoided, which increases the physiological plastic properties and high resistance against aggressive bio-
compatibility. This monomer can also be used as a cross-linking logical media [28,30-32]. When the polymers were placed in
agent. For this purpose, 1-chloroanthraquinone was coupled water, even after several weeks, no diffusion of monomeric
with 2-amino-2-methyl-1,3-propanediol under argon atmos- dyes out of the network was observed, as examined by UV–vis
phere and esterified with methacryloyl chloride to obtain 15 spectroscopy. This proves the covalent emplacement of the
(Scheme 1, step g), which absorbs light at 495 nm.
monomeric dyes in the polymer network. Figure 3 shows poly-
mers containing 9 and 10 after sharpening for further applica-
In the absence of argon, the formation of a side-product can be tion as iris implants.
observed. An oxidative ring closure takes place during the reac-
tion of 1-chloroanthraquinone and 2-amino-2-methyl-1,3-
propanediol, which gives 14 as a side-product in low yield
(6%). The yield can be increased by microwave assistance and
the use of higher temperatures [27]. It can be assumed that the
driving force for the ring closure is the formation of a six-
membered ring; however, the mechanism has not been investi-
gated in detail so far. Since the ring closure does not affect the
spectral properties, 14 can still be used as a polymerizable red
dye.
Figure 3: Sharpened blank of polymerized red 9 and blue 10 with
HEMA, THFMA and EGDMA (left, right).
In this work, side-products derived from ring-closure reactions
were only found when aliphatic amines with two carbon units
between the amine and hydroxy functionality were used. In the For use in medical applications like iris implants, the dyes and
synthesis of green and blue dyes, aromatic amines that cannot the corresponding polymers have to be biocompatible (ISO
build six-membered rings were utilized. However, the ring- 11979-5) [33-35]. To analyze the photosensitivity of the ma-
closure can be suppressed by using inert gas or adequate com- terials, the dye-containing polymers were preserved in a sodium
pounds such as 5-aminopentane-1-ol, for example [27].
chloride solution at 35 °C and exposed to irradiation with
visible light (3 mW/cm2 2500 h) for 100 days. As an example,
Figure 4 shows the transmission spectrum for the polymer
containing cross-linker 6. After 100 days, no changes in the
Polymerization and photosensitivity of the
synthesized monomeric dyes 6, 9 and 10
By way of example for copolymerization of two monomeric transmission spectra could be observed, which proves the
dyes, 9 or 10 were copolymerized with 2-hydroxyethyl- monomer dye stability and is also a hint toward its biocompati-
methacrylate (2-HEMA), tetrahydrofurfuryl methacrylate bility.
Figure 4: Sun-test results of 6.
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Beilstein J. Org. Chem. 2013, 9, 453–459.
Mixture of polymerizable dyes 2, 6 and 15
cated by mixing these three dyes in different ratios and different
By mixing the synthesized blue, red and green dyes (6, 15 and concentrations. The dyes show no significant changes in their
2) in different ratios, individual color shades can be realized. transmission spectra under sun-test conditions, and can be cova-
This so-called RGB-color space can also be found in other color lently emplaced into different copolymers. Thus, polymer ma-
devices, such as monitors, scanners and color printers. A broad terials that show no further diffusions of colored compounds out
visible spectrum can be covered by mixing these three colors in of the polymer networks were prepared. Containing two poly-
different ratios [19-21].
merizable functionalities, these monomeric dyes can also be
used as cross-linking agents. Because of their stability, bril-
The subjective color impression depends deeply on the used liance in color, and ability to cover a broad color spectrum,
monomer concentration [36]. For example, the red dye 15 these monomers can be used for medical applications such as
appears orange in high dilution, and the green dye 2 appears iris implants.
greenish-blue. At the same concentration, the blue dye 6 shows
the lowest transparency. By mixing these colors, most of the
Supporting Information
blue-containing nuances only slightly differ. Due to the absorp-
tion of the broadest area in the visible spectrum and the lowest
Supporting Information features detailed data on syntheses.
brilliance and transparency, the blue color masks the green and
Supporting Information File 1
Experimental details.
red one.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-48-S1.pdf]
To avoid these effects, the concentrations of the monomer solu-
tions had to be adapted. Each dye was diluted in acetone so as
to show the same transparency and color brilliance. By mixing
these solutions, a broad spectrum of colors can be created
(Figure 5).
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